Turkey views the Kurdish rebels as terrorists and wants to stop them taking more territory in Syria. It is also worried that the weapons will end up in the hands of the Kurdistan Workers Party (PKK), a group it has been fighting for decades.

The Pentagon has previously said it is "keenly aware" of Turkey's concerns, and that the US remains committed to "protecting our Nato ally".

However, the US believes the Kurdish elements of the Syrian Democratic Forces (SDF) will be essential to Raqqa's downfall.